The overall goal of this project is to understand how retroviruses adversely affect the central nervous system. We are utilizing the mouse as an animal model and have focused our attention on two coisogenic viruses FrCas, which causes an acute non-inflammatory spongiform encephalomyelopathy, and F43 which is avirulent. The two viruses differ in the sequence of their envelope proteins and studies in previous years revealed that the neurovirulence of FrCas was associated with folding instability of its envelope protein and induction of Endoplasmic Reticulum Stress. It is for these reasons that we now suspect that the brain disease induced by FrCas and related murine retroviruses represents a virus-induced protein folding disease.[unreadable] [unreadable] We previously found that misfolding of the FrCas envelope precursor polyprotein in the ER was determined by the sequence of the N-terminal third of the molecule. This region of the envelope protein encompasses the Receptor binding domain which is involved viral entry. The degree of neurovirulence as measured by the severity and distribution of lesions in the brain was directly correlated with the degree of folding instability as measured by steady-state binding of the ER chaperone BiP to the viral envelope protein and retention of the protein in the ER.. These results provided strong evidence that misfolding of this viral protein has a direct bearing on the capacity of the virus to precipitate neuropathology. [unreadable] In FY 2008 we found that the envelope precursor protein of FrCas forms heterogenous high molecular weight aggregates in the ER. The aggregation is caused by aberrant intermolecular disulfide bond formation. The cysteine residues participating in these abnormal disulfide bonds are located within a protein disulfide isomerase motif which normally is involved in the formation of a critical disulfide bond tethering the surface glycoprotein to the viral transmembrane protein.[unreadable] Since the folding instability of the envelope protein in the ER was determined by the sequence of the Receptor Binding Domain, we went on to study the role of the receptor in the formation of this critical SU-TM disulfide bond. The viral receptor is a cationic amino acid transporter, CAT1. Using HeLA cells, which lack the viral receptor, we found that expression of the mouse CAT1 gene modulated the formation of this disulfide bond. This suggests that CAT1 may function as a molecular chaperone for the viral envelope protein.